If you have a copy of Sonar LE somewhere, you might want to register that and use the LE to upgrade to X3 basic instead of using your 8.5 Pro for the upgrade.
This way, you can still upgrade your 8.5 Pro to current or future Pro version. You get better upgrade price this way.
I'm leaving the plugins out - you're aware X3 Pro comes with many full versions (Dim Pro, Rapture, etc) that needs to be upgraded in prior Pro version.
Going X3 basic will lose the SMPTE/MTC and Surround mixing features.
Pro Channel - since 8.5 Pro don't have it.. not having it may not effect your work flow, if you already have your personal channel strip preference. On the other hand, having the Pro Channel may enhance your work flow - X3 Pro includes all the modules (produced by Cakewalk) that we have to buy in X1 / X2.
...Melodyne Essential is included in Studio / Pro, missing in X3 basic.